FLIGHT, 7 May 1954
THE TWO Rs . . .
run on October 15th, a number of weaknesses were revealed, and
it was not until July 1934 that a 100-hr type-test could be com
pleted. This first unit gave 625 h.p. at 2,500 r.p.m. for take-off,
and 790 h.p. at 12,000ft. Rated boost was plus 2 lb, and weight
1,177 1b.
In The Magic of a Name, Harold Nockolds relates how early bench
tests resulted in persistent cracking of cylinder jackets and failures
of the double-helical reduction gear. The substitution of straight
spur-gears cured the reduction-gear trouble, but strengthening
of the big integral cylinder blocks and top half of the crankcase
was not regarded as the complete answer to the cylinder-jacket
defects. Experiments with different types of cylinder heads were
made, and the integral cylinder block and upper half of the crank-
case were replaced by separate castings for the two components.
The Rolls-Royce engineers then justifiably felt that they were
on the right road, but an attempt in 1935 to pass a 50-hr
civil type-test with the Merlin C (as the engine incorporating
these changes was called) ended in failure, and it was not until
December of that year that the test was completed. The rating
was 955 b.h.p. at 2,600 r.p.m. at 11,000ft, with a maximum output
of 1,045 b.h.p. at 3,000 r.p.m. at 12,000ft, and the engine (Merlin
F) was put into production as the Merlin I.
In September 1937 Flight published the first description of the
Mks I and II, remarking that engines of this series had then
flown for over 2,000 hr and that they had shown a marked
superiority over the early Kestrels in respect of the rough treat
ment they would stand. It was disclosed that the chief difference
between the Merlin I and II (formerly Merlin G) lay in the
cylinder heads. Whereas in the Merlin I these were of the
detachable "ramp" type, the Merlin II had blocks and heads cast
in a unit, following earlier practice. Both models had four valves
per cylinder, each with two concentric return springs. There
were two sodium-cooled exhaust valves on the outside of the head
and two inlet valves on the inside. On the Merlin II all four
were parallel to the centre-line of the block, but the two inlet
valves in the detachable head of the Merlin I were inclined at
about 45 deg to the exhaust valves. In both engines the latter
had phosphor-bronze guides, and high-silicon-chrome steel seat-
ings were screwed into the heads.
A fixed-datum automatic boost regulator maintained a con
stant induction-pipe pressure without continual reference to the
boost gauge and throttle adjustment.
The hollow crankshafts were carried in seven special lead-
bronze bearings, and the reduction gear was of 0.477:1 ratio.
Half the casing for the gearing was cast integrally with the crank-
case; in this respect the Merlin differed from the Kestrel. Oil
pumps carried on the lower half of the crankcase took their drive
from the wheelcase through an idler gear. The dry-sump system
was employed, and two scavenge pumps drained the front and
rear ends of the crankcase. The pistons and the floating steel
gudgeon-pins, which had phosphor-bronze bushes, were splash-
lubricated, a baffle in the lower half of the crankcase preventing
excess oiling.
Sandwiched between the supercharger and the crankcase at
the rear of the engine was a wheelcase from which a full comple
ment of drives was taken. The Rolls-Royce/S.U. carburettor
was of the twin-choke tube, updraught type, with a separate
diffuser to each choke placed at right angles to the airstream. The
semi-automatic, two-stage mixture-control device was operated
by air intake pressure, boost and/or a cockpit lever.
International power of the Merlin I and II was 950/990 h.p. at
2,600 r.p.m. at 12,250ft, and the maximum take-off output was
890 h.p. at 2,850 r.p.m.
When some of the first figures for the Merlin were published
in Flight during May 1937, a note was appended on the develop
ment by Rolls-Royce, Ltd., of compact "power plant" assemblies,
wherein the mounting was arranged to permit the radiator being
carried close to the crankcase. Moreover, by mounting the header
tank round the nose of the reduction gear the amount of piping
was reduced to a minimum. Advantage was taken of then-recent
research in the reduction of cooling drag by enclosing the radiator
in a low-drag cowling, wherein the cooling was done by air at
relatively low velocity, and from which the flow through the
matrix was controlled to suit various flight conditions by an
adjustable flap at the exit.
A tribute was paid also to Rolls-Royce's special experimental
flight at Hucknall, where, on April 12th, 1935, a P.V.12 engine
had first been flown in a Hawker Hart. (Other early machines
used at Hucknall were the High Speed Fury II, a Gloster Gnat-
snapper, a Hawker Horsley, a Heinkel He 70A, and a Fairey
Battle.) "Valuable work in connection widi the initial proof tests
of new engines, the development of low-drag cooling systems,
efficient exhaust systems, and flight problems associated with the
use of variable-pitch airscrews, has already been done," it was
reported.
577
During June 1937 a Merlin II, mounted in a Horsley, began
a 400-hr flight endurance test at Farnborough, and a specially
rated "racing" engine was developed from it with a view to installa
tion in the special Speed Spitfire, with which an attack on the
world's speed record was contemplated. The engine used was
a Merlin III, which differed from the Merlin II in having a
standardized de Havilland/Rotol airscrew shaft and dual
accessory-drive. It was taken from stock and was fitted with
strengthened pistons, gudgeon-pins and connecting rods to with
stand the extra load. "The power output of the standard engine,"
writes Harold Nockolds, "was 1,030 b.h.p. at 3,000 r.p.m. at
10,250ft with plus 6i lb boost. "Solely by opening the throttle,
raising the supercharger pressure, and using fuel of a higher
octane," he goes on [the petrol normally used at that time was 87
octane], "the engine was made to develop no less than 2,160 b.h.p.
at 3,200 r.p.m. with the supercharger giving 27 Ib/sq in boost.
This was a phenomenal performance, for it meant that a power-
to-weight ratio of 0.621 lb per horsepower had been achieved—
a considerable improvement on the 0.71 lb per horsepower of the
1931 R engine.
"This tremendous output, which was admittedly only attained
for a short period, nevertheless gave ample proof of the inherent
possibilities of the Merlin. But Elliott and Hives were perhaps
even more satisfied with a 15-hr endurance run at 1,800 b.h.p.,
3,200 r.p.m. and 22 lb boost accomplished during the development
period. After this they felt perfectly satisfied that the Merlin
would be capable of meeting all the demands that might be
made of it. How right they were!"
How right, indeed, for the story of Merlin development we
now have to outline is one of the most inspiring in the history of
British aero-engineering.
The Merlin II and III were installed in the Spitfire I, Defiant I,
Hurricane I, Sea Hurricane I, and Battle I, and were—as will
always be remembered—vital factors in the winning of the Battle
of Britain. The Merlin IV had pressure-water cooling in place
of the glycol cooling of the earlier models, and was developed
for installation in the Armstrong Whitworth Whitley IV bomber.
The Mk VIII, installed in the Fairey Fulmar I, was a medium-
supercharged unit rated at 1,010 h.p. at 2,850 r.p.m. at 6,750ft,
and, using 100-octane fuel, delivered 1,080 h.p. at 3,000 r.p.m. for
take-off.
The Merlin X—installed in the Halifax I, Wellington II and
Whitley V and VII—represented a very important advance in
that it had a two-speed supercharger to improve take-off, low-
altitude performance during climb or level flight, and fuel
economy under cruising conditions. The speed change was
effected through an oil-pressure system, the actual changeover
under full power taking about a second. In low gear the Merlin X
gave 1,145 h.p. at 5,250ft, and in high gear 1,010 h.p. at 17,750ft.
The Merlin XII, driving a Rotol three-blade constant-speed
airscrew, was installed in some Spitfire lis; its maximum output
was 1,150 h.p. at 3,000 r.p.m. at 14,000ft and it had a 0.477:1
reduction gear.
The next production-type engine was the Merlin XX, which,
compared with the X, delivered a greatly increased power at
height. The two units were, however, interchangeable. Describ
ing the engine in great detail during 1942, Flight remarked: "So
far-seeing and gifted were the designers in evolving the original
design that the successive types up to the latest and far more
powerful Merlin XX remain substantially the same. Even the
crankshaft and bearings, the pistons, cylinder blocks, crankcases,
are basically the same as in the original conception, though
naturally innumerable detail improvements have been effected
as constant study and experiment and metallurgical improve
ments have made possible."
The two-speed supercharger of the Merlin XX was of
improved design, incorporating a modified form of central
entry which gave a freer flow of air to the blower. The low-gear
ratio was 8.15:1 and the high gear 9.49:1. Amendments were